Filter for cooling water in a light water cooled nuclear reactor

ABSTRACT

The invention refers to a filter ( 1 ) for separating particle from cooling water in a nuclear plant, and a fuel assembly with such a filter. The filter has an inlet end ( 2 ) and an outlet end ( 3 ) and permits through-flow of the cooling water in a main flow direction (x). The filter includes a number of sheets ( 4 ) extending in the flow direction from the inlet end to the outlet end. The sheets are arranged beside each other and form passages for the cooling water. The sheets include a first portion ( 4 ′) extending from the inlet end ( 2 ), a second portion ( 4 ″) extending from the outlet end ( 3 ), and a third portion ( 4 ′″) extending between the first portion ( 4 ′) and the second portion ( 4 ″). The sheets ( 4 ) have along the first portion continuous wave-shape extending in a direction (y) transversally to the flow direction (x) and along the third portion a continuous wave-shape extending in the flow direction (x).

THE BACKGROUND OF THE INVENTION AND PRIOR ART

[0001] The present invention refers to a filter according to thepreamble of claim 1. The invention also refers to a fuel assemblyaccording to the preamble of claim 20.

[0002] The invention will be described in an application for nuclearplants for cleaning the cooling water, which flows through a nuclearreactor of a light water type, from debris and other particles. However,the invention is not limited to any particular positioning of the filterin the nuclear plant.

[0003] It is important to clean the cooling water in a nuclear plant.The purpose of the cooling water is to function as a cooling fluid and amoderator in the nuclear reactor of the nuclear plant. If debris orother particles are permitted to follow the cooling water in the core ofthe reactor, these may cause defects to the cladding of the fuel rods,which may result in such defects that nuclear fuel, i.e. uranium leaksout into the cooling water. At larger defect, the operation of thereactor has then to be interrupted and the failed fuel be replaced. Sucha replacement is time-consuming and expensive. Debris and otherparticles may of course also cause defects to other components in anuclear plant, for instance pumps.

[0004] Such debris may consist of metal chips, which are formed inconnection with different repairs of components of the plant, metalwires or other foreign particles which have reached the plant fromoutside. Particularly difficult particles are such with an elongatedshape, i.e. thin wires or chips which may have a length of down to 10mm. Such particles tend to get attached to the fuel assembly at a higherlevel, for instance to spacers. The particles are vibrating in thecooling water stream and may wear the cladding of the fuel rods so thata hole arises. At the same time it is important not to filter awayparticles which are not considered to be dangerous since all matterswhich are caught by the filter increase the pressure drop across thefilter. Such particles may for instance be blasting sand with the sizeof 1-2 mm and mineral particles, which may reach the cooling water incase of defects.

[0005] In order to solve this problem, it is known to provide some formof a filter in the lower part of the fuel assemblies, which include anumber of fuel rods and which form the core of the reactor. The coolingwater, which circulates through the reactor, passes through this lowerpart of the fuel assemblies. For instance, the bottom plate of the fuelassembly may be provided with a plurality of small holes through whichthe cooling water passes. Possible debris or other particles may thus becaught by such a filter. There are two important requirements on such afilter, on one hand it is to catch all particles which may cause defectsin the reactor in an efficient manner and on the other hand it is tohave a low flow resistance and pressure drop.

[0006] SE-B-465 192, U.S. Pat. No. 5,481,577 and U.S. Pat. No. 5,030,412disclose different such filters for catching debris in the cooling waterflowing through a nuclear reactor. SE-B-465 192 suggests holes in thebottom plate, which have different portions with centre lines that aredisplaced in relation to each other. U.S. Pat. No. 5,481,577 suggests afilter which consists of a number of sheets arranged beside each otherand forming passages for the cooling water. The passages are relativelythin but have a large width, which makes it possible for elongateddebris particles to pass. U.S. Pat. No. 5,030,412 discloses a filter,which includes a plane metal sheet, which has relatively elongatedpassages that let the cooling water through but stops possibleparticles. Upstream of the metal plate, parallel, substantially verticalsheets are provided at a small distance from each other. These parallelsheets have an intermediate curvature, which contributes to giving theparticles a desired extension transversally to the cooling water flowbefore they reach the metal sheet.

[0007] DE-U-296 15 575 discloses another filter for a fuel assembly in anuclear plant. The filter consists of a frame, in which a package ofsheets are provided beside each other in such a way that cooling waterpassages are formed between the plates. The plates have a wave-shape,which either extends in a direction transversally to the flow directionor in the flow direction.

SUMMARY OF THE INVENTION

[0008] The object of the present invention is to provide a filter, whichhas a low flow resistance and which may separate particles from a fluidin an efficient manner. Furthermore, it is aimed at a filter that can bemanufactured at a low cost.

[0009] This object is obtained by the device initially defined, which ischaracterised by the combination of the features that said sheets alongthe first portion have a wave-shape extending in a directiontransversally to the flow direction and that said sheets along the thirdportion have a wave-shape extending in the flow direction.

[0010] Such a filter, which may be manufactured of relatively thinsheets, has a low flow resistance since it does not require any loosecomponents, connection members or the like which extend in the flowpath. Thanks to the defined wave-shape of the sheets, a plurality ofseparate channels, which are arranged beside each other and which enablean efficient catching of particles in the cooling water, are obtained.The applicant has found that elongated particles are transported in thecooling water flow with an extension which substantially extendstransversally to the flow direction. Such particles may consequently becaught by the filter by means of the wave-shape of the first portion.The particles, which for any reason are transported with an extensionextending substantially in parallel with the flow direction, will becaught by the filter by means of the wave-shape of the third portion.The wave-shape of the sheets into directions perpendicular to each otheralso gives a high strength to the filter, which thus may be madeself-supporting and may be mounted in for instance a fuel assemblywithout any frame extending around the sheets.

[0011] According to an embodiment of the invention, said wave-shapes arecontinuous, i.e. without any sharp transitions. In such a way, thestrength is further improved and at the same time the manufacturing ofthe sheets by form-pressing is facilitated.

[0012] According to a further embodiment of the invention, said sheetshave, also along the second portion a wave-shape in said directiontransversally to the flow direction. Elongated particles which for anyreason pass the third portion will have an extension transversally tothe flow direction and thus be stopped by means of the wave-shape of thesecond portion. Advantageously, said sheets are along the first portionarranged beside each other in such a way that substantially each pair ofadjacent sheets abuts each other at valleys and ridges, respectively, ofsaid wave-shape, wherein each passage between two adjacent sheets formsa plurality of inlet channels arranged beside each other. Furthermore,said sheets may, along the second portion, also be arranged beside eachother in such a way that substantially each pair of adjacent sheetsabuts each other at valleys and ridges, respectively, of saidwave-shape, wherein each passage between two adjacent sheets forms aplurality of outlet channels arranged beside each other.

[0013] According to a further embodiment of the invention, said sheetsare connected to each other at at least one point at said valleys andridges, respectively, preferably by means of a fuse weld, for instancein the form of a spot weld. A further simple welding method is to weldthe sheet edges with or without supply of additional weld material. Theheat may be supplied by means of for instance an arc (TIG), laser orelectron beam. The sheets may also be welded or brazed together withadditional material. By means of such a connection of the sheets, apackage of attached sheets which are self-supporting is obtained, i.e.no further members are necessary for keeping the package of sheetstogether.

[0014] According to a further embodiment of the invention, substantiallyeach wave of said wave-shape of the first portion and the second portionhas a maximum amplitude, wherein the maximum amplitude decreasescontinuously in the direction towards the third portion. Advantageously,this maximum amplitude is substantially zero at the transition to thethird portion.

[0015] According to a further embodiment of the invention, each inletchannel has substantially the same flow area as each outlet channel. Thecentre line of substantially each inlet channel may advantageously besubstantially concentric with the centre line of a respectivecorresponding outlet channel.

[0016] According to a further embodiment of the invention, the thirdportion forms an intermediate channel between two adjacent sheets, whichis arranged to convey the cooling water between the first portion andthe second portion. Preferably, the sheets along substantially the wholethird portion are arranged at a distance from each other, i.e. they donot abut each other. In this connection said sheets may along the thirdportion at least include a part portion extending substantially inparallel with said direction transversally to the flow direction.

[0017] According to a further embodiment of the invention, the thirdportion includes projections extending into the intermediate channel. Bymeans of such projections, possible particles, which have penetrated thefilter through the inlet channel and are transported with an extensiontransversally to the flow direction, may in an efficient manner beprevented from flowing through the filter. Advantageously, saidprojections are arranged along a line extending substantially inparallel with said direction transversally to the flow direction,wherein one such part portion is arranged on each side of theprojections.

[0018] According to a further embodiment of the invention, said centreline of the inlet channel and the outlet channel extends between twoadjacent projections of the third portion. In such a way a possibleparticle has to deviate further from its path in order to be able topass through the third channel. Such projections, which may be formed bymeans of a plastic deformation of the sheet and/or include a tab cutfrom the sheet, will hinder elongated particles extending transversallyto the flow direction from passing through the intermediate channel.

[0019] The object is also obtained by the fuel assembly initiallydefined, which is characterised by the combination of features that saidsheets along the first portion have a wave-shape extending in adirection transversally to the flow direction and that said sheets alongthe third portion have a wave-shape extending in the flow direction. Thefilter and the bottom part may then be arranged to convey the coolingwater into said interspace.

BRIEF DESCRIPTION OF THE DRAWINGS

[0020] The present invention is now to be explained more closely bymeans of various embodiments, which are described as examples, and withreference to the drawings attached.

[0021]FIG. 1 discloses a perspective view of a filter according to theinvention.

[0022]FIG. 2 discloses a side view of a filter according to theinvention.

[0023]FIG. 3 discloses another side view of a filter according to theinvention.

[0024]FIG. 4 discloses a schematic side view of a fuel assembly j) for aboiling water reactor.

[0025]FIG. 5 discloses a schematic side view of a fuel assembly for apressure water reactor.

[0026]FIG. 6 discloses a side view of a bottom part of the fuel assemblyin FIG. 4.

[0027]FIG. 7 discloses a view from above of the bottom part in FIG. 6.

[0028]FIG. 8 discloses a sectional view from the side of the bottom partin FIG. 6 along the lines A-A in FIG. 7.

DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS OF THE INVENTION

[0029] FIGS. 1-3 disclose a filter 1 for separating particles from thecooling water in a nuclear plant. The filter 1 has an inlet end 2 and anoutlet end 3. The cooling water may thus flow through the filter 1 fromthe inlet end 2 to the outlet end 3 in a main flow direction x.

[0030] The filter 1 includes a number of sheets 4, which extendsubstantially in the flow direction x from the inlet end 2 to the outletend 3. The sheets 4 are arranged beside each other and form a package ofattached sheets 4. The sheets 4 are preferably manufactured of ametallic material, for instance stainless steel. The sheets 4 have afirst portion 4′, which extends from the inlet end 2 in the flowdirection x and which has a wave-shape in a direction y extendingtransversally to the flow direction x. The sheets 4 also have a secondportion 4″, which extends from the outlet end 3 opposite to the flowdirection x and which has a wave-shape in a direction y extendingtransversally to the flow direction x. Furthermore, the sheets 4 have athird portion 4′″, which extends in the flow direction x between thefirst portion 4′ and the second portion 4″. The third portion 4′″ has awave-shape in the flow direction x, i.e. the waves of the third portion4′″ extend transversally to the waves of the first portion 4′ and thesecond portion 4″.

[0031] The sheets 4 are arranged beside each other and form passages forthe cooling water through the filter 1 from the inlet end 2 to theoutlet end 3. Thanks to the wave-shape of the first portion 4′, thesheets 4 may be provided beside each other in such a way thatsubstantially each pair of adjacent sheets abuts each other at abutmentpoints or possibly along abutment lines extending along the ridges inthe flow direction x. In such a way each passage between two adjacentsheets 4 will form a plurality of channels arranged beside each otherbetween adjacent abutment lines. Such an abutment is also obtainedadjacent pairs of sheets 4 along the length of the second portion 4″.

[0032] The sheets 4 are attached to each other by means of one orseveral welds, which are applied at the abutment. In such a way, thepackage of sheets 4 may be kept together to a self-supporting structure.It is an advantage that the sheets have many attachment points. Theparticles, which by wear risk to damage the fuel, may of course damagethe filter. By means of many redundant attachment points, the structureand assembly of the filter is not risked. However, it is possible tokeep the package together in another way than by weld joints. Forinstance, various types of clamping members may be arranged around thepackage of sheets and press these together against each other along saidabutment lines.

[0033] The channels of the first portion 4′ form the inlet channels 6for the cooling water flowing through the filter 1. In the same way, thechannels of the second portion 4″ form the outlet channels 7 leading thecooling water out of the filter 1. As appears from FIG. 3, the inletchannels 6 have a longer length than the outlet channels 7 in the flowdirection x. However, it is to be noted that the inlet channels 6 alsomay be equally long as the outlet channels 7 or even shorter than theoutlet channels 7. As appears from primarily FIGS. 1 and 3, the inletchannel 6 has a centre line, which is substantially concentric with thecentre line of the outlet channel 7 for each channel in the filter 1,i.e. there is an outlet channel 7 located substantially opposite to aninlet channel 6. Seen in the flow direction x, the inlet channels 6 andthe outlet channels 7 have a larger width in the direction y than in adirection z, which is perpendicular to the direction y and the flowdirection x and which extends transversally through the plates 4substantially perpendicular to the extension plane x, y of the sheets 4.The width of each inlet channel 6 and outlet channel 7 in the directiony may be in the order of 8-11 mm, for instance 10 mm, and the width ofeach inlet channel 6 and outlet channel 7 in the direction z may be inthe order of 3-6 mm, for instance 5,5 mm. The total width of the filter1 in the flow direction x may be in the order of 20-30 mm, for instance25 or 28 mm, wherein the inlet channel 6 has a length in the order of6-8 mm and the outlet channel 7 has a length in the order of 3-8 mm. Itmay be an advantage from a manufacturing point of view if the inlet andthe outlet are symmetrical.

[0034] Each channel also includes an intermediate channel 8, whichextends between the inlet channel 6 and the outlet channel 7 and isarranged to convey the cooling water between the first portion 4′ andthe second portion 4″. The intermediate channels 8 are formed by thethird portion 4″ of said sheets 4. The third portion 4′″ connects thefirst portion 4′ and the second portion 4″. Since the third portion 4′″of each sheet 4 also includes a wave-shape, which is perpendicular tothe wave-shape of the first portion 4′ and the second portion 4″, theintermediate channel 8 will extend in a curved path between the inletchannel 6 and the outlet channel 7. The curved path will thus have acurvature in a plane including the flow direction x and the direction z.

[0035] The intermediate channel 8 does not include channels that aredelimited from each other in the same way as the inlet channels 6 andthe outlet channels 7. Separate intermediate channels are defined partlyby projections 9 of the sheets 4. The projections 9 are in theembodiment disclosed shaped as plastically deformed buckles of the sheet4. These buckles are positioned at the same distance from each otheralong a straight line as the ridges of the wave-shape of the firstportion 4′ and the second portion 4″. Advantageously, the projections 9are synchronised with the ridges of the first portion 4′ and the secondportion 4″, see FIG. 1, but may also be displaced by half a wavelengthin relation to the ridges of the first portion 4′ and the second portion4″, see FIG. 2. By such a design, elongated particles, which havepenetrated an inlet channel 6, are prevented in a secure manner frompassing through the intermediate channel 8. It is to be noted that allsheets 4 except for the uppermost one, see FIG. 3, are provided withsuch projections 9. The projections 9 may be designed in many differentways, for instance they can be formed by tabs being cut from the sheet4.

[0036] The third portion 4′″ includes two part portions 10, 11, whichextend substantially in parallel with the direction y andperpendicularly to the flow direction x. The part portions 10 and 11 arearranged on a respective side of the line of projections 9.

[0037] The filter 1 is suitable especially but not exclusively formounting in a fuel assembly for a nuclear plant. FIGS. 4 and 5 disclosetwo different types of fuel assemblies 15 and 16, respectively, whichare suitable for including the filter 1. FIG. 4 discloses a fuelassembly 15 intended for a boiling water reactor, BWR, and including anupper part 20 and a bottom part 21. A number of fuel rods 22 areprovided between the upper part 20 and the bottom part 21. The fuel rodsare in their lower ends connected to the bottom part 21 and in theirupper end to the upper part 20. Furthermore, the fuel assembly 15includes spacers 23, which are distributed along the length of the fuelrod 22 and which serve the purpose of keeping the fuel rods 22 in adesired position. Moreover, the fuel assembly 15 includes a casing 24,which extends between the upper part 20 and the bottom part 21 and whichencloses all the fuel rods 22. A filter 1 according to the descriptionabove is arranged in the bottom part 21. The filter 1 is schematicallyindicated in FIG. 4. The fuel assembly 15 is arranged to permit coolingwater to flow into the fuel assembly through the bottom part 21 and inbetween the fuel rods 22.

[0038] The bottom part 21 is disclosed more closely in FIGS. 6-8. FromFIGS. 7 and 8 appears that the fuel assembly includes four filters 1,which are located in a respective substantially square opening 27 of thebottom part 21. The filters 1 are provided in parallel with each otherand all cooling water flowing into the bottom part 21 via an inletorifice 28 will flow through any of the filters 1. It is to be notedthat the bottom part 21 also could include another number of openings 27and filters 1, for instance one single larger opening 27 with only onefilter 1.

[0039]FIG. 5 discloses a fuel assembly 16 for a pressure water reactor,PWR. The fuel assembly 16 also includes an upper part 30, a bottom part31 and a number fuel rods 32. In addition, the fuel assembly 16 includesa number of guide tubes 33 extending between and connecting the bottompart 31 and the upper part 30. The fuel rods 32 are held by means ofspacers 34 which are connected with the guide tubes 33. The filter 1 isalso in this case arranged in the bottom part 31 and schematicallyindicated in FIG. 5. All cooling water flowing into the fuel assemblybetween the fuel rods 32 will thus flow through the filter 1. In theembodiment disclosed in FIG. 5, the fuel assembly 16 includes only onesingle filter covering the whole area of the bottom part 31 seen in ahorizontal section but also in this case the fuel assembly 16 may ofcourse include more filters 1, for instance four.

[0040] The invention is not limited to the embodiments disclosed but maybe varied and modified within the scope of the following claims.

1. A filter for separating particles from cooling water in a nuclearplant of a light water type, wherein the filter (1) has an inlet end (2)and an outlet end (3) and is arranged to permit through-flow of thecooling water in a main flow direction (x) from the inlet end (2) to theoutlet end (3), wherein the filter (1) includes a number of sheets (4),which extend substantially in the flow direction (x) from the inlet end(2) to the outlet end (3), wherein said sheets (4) are arranged besideeach other and form passages for the cooling water through the filter(1) from the inlet end (2) to the outlet end (3), and wherein saidsheets includes a first portion (4′), which extends from the inlet end(2), a second portion (4″), which extends from the outlet end (3), and athird portion (4′″), which extends between the first portion (4′) andthe second portion (4″), characterised by the combination of features:that said sheets (4) along the first portion (4′) have a wave-shapeextending in a direction (y) transversally to the flow direction (x) andthat said sheets (4) along the third portion (4′″) have a wave-shapeextending in the flow direction (x).
 2. A filter according to claim 1,characterised in that said wave-shapes are continuous, i.e. without anysharp transitions.
 3. A filter according to claim 1, characterised inthat said sheets along the second portion have a wave-shape in saiddirection (y) transversally to the flow direction (x).
 4. A filteraccording to any one of claims 1 to 3, characterised in that said sheetsalong the first portion are arranged beside each other in such a waythat substantially each pair of adjacent sheets (4) abuts each other atvalleys and ridges, respectively, of said wave-shape, wherein eachpassage between two adjacent sheets forms a plurality of inlet channels(6) arranged beside each other.
 5. A filter according to claim 4,characterised in that said sheets (4) along the second portion (4″) arearranged beside each other in such a way that substantially each pair ofadjacent sheets (4) abuts each other at valleys and ridges,respectively, of said wave-shape, wherein each passage between twoadjacent sheets forms a plurality of outlet channels (7) arranged besideeach other.
 6. A filter according to any one of claims 4 and 5,characterised in that said sheets (4) are connected to each other at atleast one point at said valleys and ridges, respectively.
 7. A filteraccording to claim 6, characterised in that said sheets (4) areconnected to each other by means of a fuse weld where the sheets (4)abut each other.
 8. A filter according to claim 6, characterised in thatsaid sheets (4) are connected to each other by means of a spot weldwhere the sheets (4) abut each other.
 9. A filter according to any oneof the preceding claims, characterised in that substantially each waveof said wave-shape of the first portion (4′) has a maximum amplitude,wherein the maximum amplitude decreases continuously in the direction(x) towards the third portion (4′″).
 10. A filter according to claims 3and 9, characterised in that substantially each wave of said wave-shapeof the second portion (4″) has a maximum amplitude, wherein the maximumamplitude decreases continuously in the direction (x) towards the thirdportion (4′″).
 11. A filter according to any one of claims 9 and 10,characterised in that said maximum amplitude is substantially zero atthe transition to the third portion (4′).
 12. A filter according to anyone of claims 5 to 11, characterised in that each inlet channel (6) hassubstantially the same flow area as each outlet channel (7).
 13. Afilter according to any one of claims 5 to 13, characterised in that thecentre line of substantially each inlet channel (6) is substantiallyconcentric with the centre line of a respective corresponding outletchannel (7).
 14. A filter according to any one of the preceding claims,characterised in that the third portion forms an intermediate channel(8) between two adjacent sheets.
 15. A filter according to claim 14,characterised in that said sheets along the third portion (4′″) at leastincludes a part portion (10, 11) extending substantially in parallelwith said direction (y) transversally to the flow direction (x).
 16. Afilter according to any one of claims 14 and 15, characterised in thatthe third portion (4′″) includes projections (9) extending into theintermediate channel (8).
 17. A filter according to claims 15 and 16,characterised in that said projections (9) are arranged along a lineextending substantially in parallel with the said directiontransversally to the flow direction (x), wherein one such part portion(10, 11) is arranged on each side of the projections (9).
 18. A filteraccording to claims 13 and 17, characterised in that said centre line ofthe inlet channel (6) and the outlet channel (7) extends between twoadjacent projections (9) of the third portion (4′″).
 19. A filteraccording to any one of claims 16 to 18, characterised in that saidprojections (9) are formed through plastic deformation of the sheet. 20.A filter according to any one of claims 16 to 19, characterised in thatsaid projections (9) include a tab, which is cut from the sheet.
 21. Afuel assembly for e nuclear plant of a light water type, wherein thefuel assembly (15, 16) includes a bottom part (21, 31) a top part (20,30) and a plurality of fuel rods (22, 32) arranged beside each other andwith an interspace between each other between the bottom part and thetop part of the fuel assembly, wherein the bottom part (21, 31) includesa filter (1) for separating particles from cooling water circulatedthrough the fuel assembly, wherein the filter (1) has an inlet end (2)and an outlet end (3) and is arranged to permit through-flow of thecooling water in a main flow direction (x) from the inlet end (2) to theoutlet end (3), wherein the filter (1) includes a number of sheets (4),which extend substantially in the flow direction (x) from the inlet end(2) to the outlet end (3), wherein said sheets (4) are arranged besideeach other and form passages for the cooling water through the filter(1) from the inlet end (2) to the outlet end (3), and wherein saidsheets include a first portion (4′), which extends from the inlet end(2), a second portion (4″), which extends from the outlet end (3), and athird portion (4′″), which extends between the first portion (4′) andthe second portion (4″), characterised by the combination of features:that said sheets (4) along the first portion (4′) have a wave-shapeextending in a direction (y) transversally to the flow direction (x) andthat said sheets (4) along the third portion (4′″) have a wave-shapeextending in the flow direction (x).
 22. A fuel assembly according toclaim 21, characterised in that the filter (1) and the bottom part (21,31) are arranged to guide the cooling water into said interspace.
 23. Afuel assembly according to any one of claims 21 and 22, characterised inthat the filter includes features from any one of claims 2-20.